1. Field of the Invention
The present invention relates to an apparatus for measuring a seat weight including a weight of a passenger sitting on an automotive seat and more specifically to a seat weight measuring apparatus with advantages of being able to reduce costs or improve reliability of a circuit portion that determines the weight of the sitting passenger.
2. Description of a Related Art
Automobiles are equipped with seat belts and airbags to secure safety for passengers. In recent years, there is a trend for controlling the operation of such safety devices according to the weight of a passenger for improved performance of seat belts and airbags. For example, the amount of gas introduced into the airbag or an airbag inflating speed and a pre-tension of the seat belt are adjusted according to the weight of a passenger. For that purpose, means are needed for measuring the weight of a passenger sitting on a seat. An example of such means, which has been proposed by the applicant of this invention, involves arranging load sensors (strain gauges) at four corners of the bottom of a seat, measuring vertical loads acting on the strain gauges as voltages by electric circuits, and summing these voltages to determine the seat weight including the weight of a passenger.
First, the basic configuration of the electric circuits used in the above seat weight measuring apparatus will be described.
FIG. 9 shows a circuit configuration for one load sensor.
This sensor circuit 20 includes a power supply 21 for applying a predetermined voltage, a load sensor 22 of a full bridge configuration having four resistors R1-R4, an amplifier 23 for Amplifying the voltage from the load sensor 22, and a positive polarity wire 24 and a negative polarity wire 25 for connecting the load sensor 22 to the amplifier 23. This load sensor 22 has four strain gauges (not shown) attached to a sensor plate (not shown) that deflects when applied with a load, with a resistance of each resistor R1-R4 changing according to the amount of strain of each strain gauge. The load sensor 22 is formed as a full bridge circuit including four resistors R1-R4 so that it can cancel the strain change due to temperature change and the voltage change of the power supply 21.
FIG. 10 shows a circuit configuration for all load sensors incorporated in the seat weight measuring apparatus.
The sensor circuit 26 includes load sensors 22a-22d located at four corners of the bottom of a seat, amplifiers 23a-23d connected to the respective load sensors 22a-22d, positive polarity wires 24a-24d and negative polarity wires 25a-25d for connecting the load sensors 22a-22d to the amplifiers 23a-23d, an adder 27 for adding up voltages output from the amplifiers 23a-23d, and a seat weight calculator 28 for determining the weight of the seat based on the voltage from the adder 27. Each of the load sensors 22a-22d is supplied with a predetermined voltage.
In FIGS. 9 and 10, the strain gauges forming the load sensor 22 are deformed by a vertical load from the seat (not shown). The amounts of strain of the strain gauges determine the resistances of the resistors R1-R4 of the load sensor 22. When a predetermined voltage is applied from the power supply 21 to the load sensor 22 as a resistor circuit, a voltage as determined by the supply voltage and the resistances (R1-R4) is applied to the amplifier 23, which amplifies the voltage before outputting it to the adder 27. The adder 27 adds up voltages from the four amplifiers 23a-23d and outputs the sum voltage to the seat weight calculator 28. The seat weight calculator 28 calculates the weight of the seat according to the voltage from the adder 27.
In this way, the load from the seat can be measured by the load sensors located at four corners of the bottom of the seat.
In the seat weight measuring apparatus such as shown in FIGS. 9 and 10, however, each of the load sensors requires one amplifier and two wires (for a positive polarity and a negative polarity). Thus, a total of four amplifiers and eight wires are needed, giving rise to a problem of increased circuit and wiring costs. As the number of constitutional components increases, the probability of parts failure also increases, deteriorating the overall reliability of the sensor.
Japanese Patent Publication No. JP-A-10-297334 discloses a passenger weight detection apparatus that calculates a gravity center of a loaded seat based on a seat weight detected from a load acting on a front part of the seat and a seat weight detected from a load acting on a rear part of the seat and which corrects the seat weight according to the calculated gravity center and then, based on the corrected seat weight, estimates the weight of a passenger.
This official gazette, however, does not disclose a circuit configuration of the passenger weight detection apparatus. If a circuit configuration such as shown in FIGS. 9 and 10 is used, this apparatus still has drawbacks of increased circuit and wiring costs and, because of a large number of constitutional parts, increased probability of parts failure which in turn degrades the overall reliability of the sensor.